This invention relates in general to electrophotographic imaging members and, more specifically, to charge transport layers comprising a cross linked matrix derived from an aromatic polymer.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrophotographic imaging process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, entails placing a uniform electrostatic charge on a photoconductive imaging member, exposing the imaging member to a light and shadow image to dissipate the charge on the areas of the imaging member exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. In the Charge Area Development (CAD) scheme, the toner will normally be attracted to those areas of the imaging member which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper. The transferred image may subsequently be permanently affixed to the substrate by heat, pressure, a combination of heat and pressure, or other suitable fixing means such as solvent or overcoating treatment. Imaging members for electrophotographic imaging systems comprising selenium alloys vacuum deposited on substrates are known. Imaging members have also been prepared by coating substrates with photoconductive particles dispersed in an organic film forming binder. Coating of rigid drum substrates has been effected by various techniques such as spraying, dip coating, vacuum evaporation, and the like. Flexible imaging members can also be manufactured by processes that entail coating a flexible substrate with the desired photoconducting material.
Some photoresponsive imaging members consist of a homogeneous layer of a single material such as vitreous selenium, and others comprise composite layered devices containing a dispersion of a photoconductive composition. An example of a composite xerographic photoconductive member is described in U.S. Pat. No. 3,121,006, which discloses finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. Imaging members prepared according to the teachings of this patent contain a binder layer with particles of zinc oxide uniformly dispersed therein coated on a paper backing. The binders disclosed in this patent include materials such as polycarbonate resins, polyester resins, polyamide resins, and the like.
Photoreceptor materials comprising inorganic or organic materials wherein the charge generating and charge transport functions are performed by discrete contiguous layers are also known. Additionally, layered photoreceptor members are disclosed in the prior art, including photoreceptors having an overcoat layer of an electrically insulating polymeric material. Other layered photoresponsive devices have been disclosed, including those comprising separate photogenerating layers and charge transport layers as described in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Deposition of charge on the photoreceptor surface by bias charging rolls (BCR) is usually accompanied by significant degradation of the charge transport layer. This degradation is believed to be caused by plasma generated in the contact zone between the charging roll and the photoreceptor at breakdown electric fields. More specifically, bias roll charging of organic photoreceptors, particularly under alternating current conditions with the positive portion of the wave unfiltered, leads to significant degradation of the photoreceptor surface and undesirable reduction of the transport layer thickness. This degradation limits the useful life of the photoreceptor and is one reason why the use of bias charging rolls is currently limited to low volume printers and copiers. Preliminary test results indicate that overcoating of a charge transport layer with a cross linked charge transport polymer improves the resistance of the photoreceptor surface to BCR degradation. However, the use of such overcoat would require yet another coating step which can reduce production yields. Moreover, the overcoat itself often does not adhere to a small molecule/binder polymer transport layer underneath. Further, if a cross linked charge transport polymer is used as transport layer, this material may not have sufficient carrier mobility.